At this point the clients request for a quote is very vague. It sounds like I should go discuss it with them to pick up on more details, examine this trailer, and see how they intend to make it work.
Now here's what I would do and what I will recommend to the client...
I would go with a Sign Comp, or similar banner tension frame mounted to the trailer. I would line the trailer inside that frame with pink foam insulation. Then I would mount a 13 oz. banner measuring more like 7.5' x 40' in the frame.
However, the reason I ask is because we have done upwards of $15,000 worth of work for these guys in the last year and a half. The business has been in operation for years, but recently moved to this much larger facility, remodeled, bought brand new signage, etc. and they are kinda tight on the budget now. I know this because I have a solid relationship with the manager and cost is definitely an issue. This is still a small, family owned, local business so they don't have a ton of capital to work with until this new location starts paying for itself so I completely understand. They have another sign left behind from the business that used to be there and after almost two years in this new building they still haven't removed the old sign, had it replaced, refaced or anything and it has nothing to do with their business as the sign says "Antiques". They also have a high dollar order of presentation folders on hold temporarily until they get enough money in the advertising budget to afford them.
So as much as I want to entertain the idea of just letting them strap a giant banner to the side of this trailer, I know the right way to do it would be a nice tension frame, but if they insist on the giant banner I will mention the mesh and not the scrim. If they find out the mesh is right for the job due to weight, but realize that it will have less visual impact from the highway, then maybe I can steer them toward the tension frame.